Structural panels are used for many applications, especially for use as core materials. For certain applications, it is desirable that the panel have both rigidity and structural strength and that it also be relatively lightweight. In an attempt to provide such panels, various techniques have been developed for providing a core of rigid material having a relatively low density sandwiched between skins of material that have relatively high densities.
In a series of patents granted to Walter Smarook and including U.S. Pat. Nos. 3,765,810, 3,919,378, 3,919,379, 3,919,380, 3,919,381, 3,919,382, 3,919,445, 3,919,446, 4,070,515, and 4,148,954, there is disclosed a process and apparatus for expanding a thermoformable material to form a panel. Specifically, a sheet of thermoformable material is disposed between two platens having a plurality of vented openings on the working surface thereof. The platens are heated to a temperature of at least 70.degree. C. at which point the material becomes tacky and is bonded to the platen by hot tack adhesion. The platens are then moved apart expanding the material in such a manner that, at the locations of the vented openings, voids are formed in the material. The process disclosed in these patents is limited to relatively small panels. As defined in these patents, the term "thermoformable" means a material that is solid at 25.degree. C. but can be reshaped or reformed above some higher temperature.
U.S. Pat. Nos. 4,113,909 and 4,164,389 describe a manner in which such a process can be applied to the formation of large sheets, e.g., 5 feet.times.10 feet sheets. Expanded panels can be made having voids on both sides, and also with one side having a continuous surface and the other containing voids.
U.S. Pat. No. 4,315,051 describes a process for producing a clear surface on expanded thermoformable material. A film layer of non-bondable plastic material is disposed adjacent the clear plastic material to be expanded. After the expansion process, the film layer is removed leaving a clear, smooth surface.
However, a major drawback attendant upon such structural panels, formed by the heretofore known processes and techniques, is that the panels have been expensive to produce due to the amount of resin material used in forming the panels so as to limit their commercial utility . As much as about 85 percent of the cost of forming the panels has been found to be attributable to material costs for making such panels, thereby rendering such panels less than competitive in many applications, as structural materials. Additionally, the presence of the relatively large amount of resin material affects adversely the weight of the panel itself. It can thus be seen that there is a substantial need for structural panels that can be formed at a relatively lower cost by using less material while maintaining sufficient strength and load-bearing capability without an attendant weight penalty.